AIMS: Carbohydrate-deficient transferrin (%CDT) is a well-established and highly specific biomarker for sustained heavy consumption of alcohol. However, in pregnant women, the specificity of this biomarker might be affected by advanced gestational age, even after accounting for increased transferrin concentrations in pregnancy. The goal of this prospective study was to assess the variability in %CDT during pregnancy among alcohol-abstaining patients.METHODS: Patients were recruited during one of the first prenatal care visits and followed-up to term. Abstinence was confirmed by maternal self-report and by alcohol biomarkers. Biomarkers assessed in the mother included serum gamma-glutamyltranspeptidase, urine ethyl glucuronide and ethyl sulfate, and whole blood phosphatidylethanol (PEth). In addition, PEth was measured in a dry blood spot card obtained from a newborn. For %CDT analysis, serum samples were collected at baseline and at term and analyzed by an internationally validated high-performance liquid chromatography and spectrophotometric detection method.RESULTS: At recruitment (mean gestational age 22.6 ± 7.3 weeks), the mean %CDT concentration was 1.49 ± 0.30%, while at term, it increased to 1.67 ± 0.28% (P = 0.001). Using a conventional cutoff concentration %CDT >1.7%, 22.9 and 45.7% of the sample would be classified as 'positive' for this biomarker at recruitment and at term, respectively (P = 0.011 ).CONCLUSION: These results suggest that a conventional cutoff of 1.7% might be too low for pregnant women and would generate false-positive results. We propose that %CDT >2.0% be used as a cutoff concentration indicative of alcohol exposure in pregnant women. The sensitivity of %CDT at this cutoff for heavy drinking during pregnancy needs to be assessed further.